Catalytic reforming is one of the basic petroleum refining processes for upgrading light hydrocarbon feedstocks, frequently referred to as naphtha feedstocks. Naphtha feedstocks can include paraffins, isoparaffins, olefins, cycloparaffines, aromatics, and naphthenes. Generally, naphtha feedstocks boil up to 450° F. and contain hydrocarbons with molecular weights from 100 to 250 g/mol. Products from catalytic reforming of naphtha feedstocks can include gasoline for use in automobiles, aromatics such as benzene, toluene, xylene and ethylbenzene for use as solvents and chemicals, and hydrogen for use in various refining processes. Reactions typically involved in catalytic reforming include dehydrocylization, isomerization, dehydrogenation, dealkylation, and hydrocracking. Dehydrocyclization and dehydrogenation of linear and slightly branched alkanes and dehydrogenation of cycloalkanes lead to the production of aromatics and are often desired reactions in reforming. Dealkylation and hydrocracking are generally undesirable due to the low value of the resulting light hydrocarbon products.
Catalysts commonly used in commercial reforming reactions often include inorganic oxides and one or more Group VIII metals, such as platinum or palladium, or a Group VIII metal plus a second catalytic metal, which acts as a promoter. The inorganic oxide can act as a support for the Group VIII metal as well as imparting some catalytic activity due to the presence of acid sites in the inorganic oxide. Crystalline inorganic oxides such as zeolites and noncrystalline inorganic oxides such as amorphous silica alumina may be used as supports. Halogens such as chlorine can be incorporated on the support to increase acid functionality. Alkali metals such as sodium, potassium, and cesium can be incorporated on the support to decrease acidity. By controlling the amount of acidity and using different catalytic metal or metals, the activity of the reforming catalyst can be tuned for optimum use of different feedstocks and/or for optimum production of desired products such as high octane gasoline.
In addition to selection of catalysts for reforming, various processes for reforming a naphtha feedstock in one or more process steps to produce higher value reformate products are known in the art. Generally one or more process steps are used to increase the RON (research octane number) and aromatic content of the naphtha feedstock. Process conditions of temperature and pressure can be varied depending on the catalyst or catalysts used, the feedstock, and the desired products. In addition, fractionation steps can be employed between steps to select for certain products and maximize yield.
Even with the advances in naphtha reforming catalysts and processes, a need still exists to develop new and improved reforming methods to provide higher liquid yield, minimize the formation of less valuable low molecule weight (C1-C4) products, and improve the economic viability of costly high pressure multi stage reforming. It has been discovered that the replacement of a conventional reforming catalyst with a beta zeolite catalyst in a penultimate stage and a silicalite catalyst in a final stage of a staged reforming process allows the process of the invention to be run at lower pressure than under conventional process conditions while yielding a product with a comparable RON, aromatics content, C5+ liquid yield, hydrogen production, and catalyst life. A clear economic benefit is realized by running a staged reforming process at lower hydrogen pressure while maintaining high yields of liquid product.